Another Solyndra already? Some worry even worse is coming.

Beacon Power, like Solyndra a recipient of a federal loan guarantee under the DOE's renewable energy program, has declared bankruptcy, leaving taxpayers to pay the tab. But the firm is tiny. Bigger ones are out there.

Beacon Power Corp., a tiny renewable energy company that was a recipient of a $43 million loan guarantee backed by the Department of Energy, declared bankruptcy on Sunday, leaving taxpayers to cover the $39.1 million in loans the company has not yet repaid.

The figure pales in comparison with the unpaid balance of nearly $535 million left by Solyndra, the California solar startup that declared bankruptcy in September, but it hinted at the potential downside of the DOE’s loan guarantee program, through which other companies have received far larger amounts, even billions of dollars.

Creator of a novel, high-tech carbon-composite flywheel to store energy and help regulate fluctuations on the electric grid, Massachusetts-based Beacon Power used the federal loan guarantee and built a 20-megawatt flywheel energy storage facility in Stephenson, N.Y. Such storage systems, including grid-scale battery storage, hold promise because they can respond more quickly than power plants can to power fluctuations in the grid, including from the addition of wind and solar power plants.

But, as was the case with Solyndra, which developed the ability to manufacture solar power panels with a cheaper material than silicon only to see the worldwide price of silicon plummet, Beacon’s ingenuity was not enough to ensure its survival.

“The current economic and political climate, the financing terms mandated by DOE, and Beacon’s recent delisting notice from Nasdaq, have together severely restricted Beacon’s access to additional investments through the equity markets,” chief executive officer F. William Capp wrote in filings with the US Bankruptcy Court in Wilmington, Del., Businessweek.com reported.

In the case of Solyndra, allegations of political favoritism and other irregularities already being investigated by Congress and the FBI have become a political sledgehammer for the GOP to use against President Obama. Whether Beacon Power becomes grist for that political mill, others are concerned that the biggest threat may lie elsewhere.

Solyndra and Beacon Power, which both won federal backing for their loans under the DOE’s $34 billion loan guarantee program for renewable energy, could be just the tip of the bad-loan-bailout iceberg, fiscal watchdog groups warn.

“Sure, we're worried about Solyndra and Beacon Power, but we're even more worried about some of the much larger loan guarantees out there,” says Autumn Hanna, a fiscal analyst with Taxpayers for Common Sense, a nonpartisan Washington taxpayers group, who has analyzed the loan guarantee program. “We have one loan guarantee for $8.3 billion for a nuclear reactor – that could be far worse” if there were a default, she says.

The Government Accountability Office in 2008 reported that the average risk of default on DOE loan guarantees for all energy projects (including nuclear and other power projects) was about 50 percent. Harking back to nuclear power industry defaults of the 1980s, a Congressional Budget Office analysis in 2003 said the default risk for new nuclear reactors was “very high,” a point the nuclear industry has repeatedly said is based on a faulty analysis.

The DOE program has so far offered conditional commitments or final loan guarantees for 32 projects valued at more than $25 billion. Of that, however, some $8.3 billion (31 percent) is being risked on just one nuclear project, Ms. Hanna's analysis shows. Another $2 billion is being sought for a uranium enrichment project in Idaho, but no loan guarantees have yet been finalized.

The DOE program’s numbers could get larger, with the Obama administration seeking $36 billion in new loan guarantee authority in its 2012 budget, primarily to accommodate nuclear power loan guarantees, she notes.

"We're working hard to ensure that the scrutiny on Solyndra carries through to these nuclear and other loan guarantees and the entire program," she says, "not just the stimulus portion of the program that focused on renewable energy."

There are signs that members of Congress may indeed be paying attention.

“I still support there being a loan guarantee program for new energy – alternative energy and nuclear energy,” Rep. Joe Barton (R) of Texas told The New York Times earlier this month. “But I do think it's fair to go in and look at the law, put more safeguards in, put caps in it.”